The use of thermal energy to destroy bodily tissue can be applied to a variety of therapeutic procedures, including the destruction of tumors. Thermal energy can be imparted to the tissue using various forms of energy, such as radio frequency electrical energy, microwave or light wave electromagnetic energy, or ultrasonic vibrational energy. Radio frequency (RF) ablation, for example, can be effected by placing one or more electrodes against or into tissue to be treated and passing high frequency electrical current into the tissue. The current can flow between closely spaced emitting electrodes or between an emitting electrode and a larger, common electrode located remotely from the tissue to be heated.
One disadvantage with these techniques is that maximum heating often occurs at or near the interface between the therapeutic tool and the tissue. In RF ablation, for example, maximum heating can occur in the tissue immediately adjacent to the emitting electrode. This can reduce the conductivity of the tissue, and in some cases, can cause water within the tissue to boil and become water vapor. As this process continues, the impedance of the tissue can increase and prevent current from entering into the surrounding tissue. Thus, conventional RF instruments are limited in the volume of tissue that can be treated.
Fluid enhanced ablation therapy, such as the SERF™ ablation technique (Saline Enhanced Radio Frequency™ ablation), can treat a greater volume of tissue than conventional RF ablation. The SERF ablation technique is described in U.S. Pat. No. 6,328,735, which is hereby incorporated by reference in its entirety. Using the SERF ablation technique, saline is passed through a needle and heated, and the heated fluid is delivered to the tissue immediately surrounding the needle. The saline helps distribute the heat developed adjacent to the needle and thereby allows a greater volume of tissue to be treated with a therapeutic dose of ablative energy. The therapy is usually completed once a target volume of tissue reaches a desired therapeutic temperature, or otherwise receives a therapeutic dose of energy.
During fluid enhanced ablation therapy, the fluid can be heated to a desired temperature in a variety of different ways. For example, the fluid can be heated remotely from the needle and then pumped into the needle at an elevated temperature. However, transferring heated fluid can result in undesirable temperature loss between the remote heater and the treatment site, as well as undesirable heating of remote portions of the patient's body. Alternatively, the fluid can be heated after it enters the needle and prior to injection into the tissue. However, it can be difficult to construct and repeatedly manufacture a heating assembly capable of disposition within the sometimes very small needle bodies used in fluid enhanced ablation. Furthermore, the needle body itself is a conductive material used to deliver energy to the treatment site, so precautions must be taken to avoid interfering with energy passed through the needle body.
Accordingly, there remains a need for improved devices and methods for heating fluid used during fluid enhanced ablation therapy.